Simulation of the Impact of Instantaneous Solar UV Radiation Enhancements on the Middle Atmosphere via UV Radiation Reconstruction

Abstract

This study reconstructed the 0.01–399.5 nm ultraviolet (UV) radiation band with a time resolution of 5 min under solar minimum conditions (18 October 2006) to investigate the effects of the solar flare event on the middle atmosphere. Five-minute resolution 0.01–399.5 nm UV radiation was used instead of daytime scale data to observe the response of the middle atmosphere to the instantaneous solar UV radiation enhancement. The results indicate that small temperature increases of 0.05 K in low latitudes were observed in the lower thermosphere and the stratosphere. The UV radiation enhancement led to an ozone increase of 0.6% in the stratosphere, which caused small temperature increases; and there is an ozone increase of up to 4% at 80 km, while a change of −2% occurred at 60–70 km and a change of −6% occurred in the low thermosphere. There was a 0.05% increase in atmospheric density above 60 km, and there was an increase of up to 0.15% at 80–90 km. The responses of the atmospheric temperature and density in the middle atmosphere to instantaneous UV radiation enhancements can therefore be captured via the UV radiation reconstruction. The simulation results were weaker than the previous study.

1. Introduction

The study of the possible response of the middle atmosphere (10–100 km: upper troposphere, stratosphere, mesosphere and lower thermosphere) to solar activity variations has been pursued now for over 80 years. Most of this work has been concentrated on possible atmospheric variations associated with the solar activity cycle (the 11-year and 27-day cycles) and solar storms (solar flare and solar proton events), including variations in ozone and temperature [1,2,3,4,5].

The response of the middle atmosphere to solar activity has always been an important topic of research for international cooperative organizations. For example, from programs such as the Middle Atmosphere Program of the International Committee of Space Physics in the 1980s to today’s Variability of the Sun and Its Terrestrial Impact [6], the response of the middle atmosphere to solar activity has been listed as the main research focus. Such studies have focused primarily on medium-term and long-term changes in the Sun, with some studies considering short-term simulations of the effect of solar storms on the middle atmosphere.

Minor radiation changes during the solar activity cycle can also impact the Earth’s climate system and significantly impact the troposphere [4]. According to satellite observations, the variation range of ultraviolet radiation in the solar activity cycle is approximately 6–8%, which is much larger than the variation range of the total solar radiation dose [7]. Unlike the absorption of visible light reaching the ground, ultraviolet radiation is directly absorbed by ozone in the stratosphere and affects the troposphere. Stratospheric ozone not only can affect the climate system by changing the radiation balance [8] but also can affect the stratospheric and tropospheric climate via chemical-radiation-dynamic feedback processes [9].

Variations in solar ultraviolet (UV) irradiance have been correlated with changes in stratospheric ozone and temperature on short time scales of days to weeks and long time scales of years to decades [10]. A series of studies were performed concerning the response of low-latitude ozone and temperature in the stratosphere and mesosphere to short-term solar ultraviolet variability associated with the 27-day rotation of the Sun based on satellite data. Keating et al. [11] found that the stratospheric ozone response with no correction for temperature effects is found to be approximately a 0.4% increase at 3 mbar for a 1.0% increase in 205-nm solar radiation, while in the mesosphere, a major systematic ozone decrease has been detected near 0.05 mbar (~70 km), with increased solar Lyman Alpha (121.6 nm) radiation (−0.14% ozone decrease for a 1% increase in solar Lyman Alpha) based on the Nimbus 7 data. Brasseur et al. [12] used a two-dimensional chemical-dynamical-radiative model of the middle atmosphere to investigate the potential changes in temperature, ozone, and other chemical constituents in response to variations in the solar ultraviolet flux, and the results showed the calculated change in the ozone column abundance from solar minimum to solar maximum conditions is of the order of 1.1–1.3% in the tropics and increases with latitude, especially in winter, to reach up to 1.5–1.7% in the polar regions. Gan et al. [13] used extended Canadian Middle Atmosphere Model (eCMAM30) simulation data from 1979 to 2010 and TIMED/SABER satellite temperature data from 2002 to 2015 to study the response of the mesospheric temperature to the 11-year solar activity cycle. Their simulation results show that there are 1–2 K/100 sfu below 80 km and 2–4 K/100 sfu between 80 and 100 km; these simulated responses are weaker than the response of the observation data. Thiéblemont et al. [14] studied the correlation between the stratospheric ozone and ultraviolet radiation in the equatorial region, which is closely related to the 27-day rotation cycle of the Sun. Pikulina et al. [15] studied the effect of solar flares on the chemical composition and ozone of the upper and middle atmosphere based on the Flare Irradiance Spectral Model-Version2 (FISM2) data and Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA). The work presented that solar flares in early September 2017 led to a significant increase in the concentrations of the reactive nitrogen and hydrogen oxides in the equatorial latitudes and southern high latitudes, while the ozone changes caused by processes that are associated with the impact of electromagnetic radiation on the Earth’s atmosphere during solar flares are not dramatic.

Space weather can have considerable impacts on the Earth and the near-Earth environment; large space weather events can have impacts on the dynamics and composition of the middle and upper atmosphere [16]. Depending on the flare’s spectral irradiance [17] and class [18], photoionization of the atmosphere by solar flares can occur at altitudes from the mesosphere up through the layers of the ionosphere. X-rays (0.1–10 nm) and EUV (10–120 nm) mainly influence the upper mesosphere and lower thermosphere [19]. Peak ionization from longer wavelength radiation (>120 nm) ranges from the upper stratosphere near 50 km to the lower thermosphere near 150 km.

Most studies have been based on the observation and simulation of the chemical composition and temperature of the middle atmosphere and ionosphere in space weather events. Pettit et al. [16] used the Whole Atmosphere Community Climate Model (WACCM) to simulate the influence of solar flares and solar proton events on the middle atmosphere in September 2005 and found that the influence of proton events on the polar stratosphere and the middle layer was stronger than that of flare events, while the influence of flares on the lower thermosphere in the equatorial region was greater. Jackman et al. [20,21,22] studied the influence of solar proton events on the middle atmosphere; the study observed NOx (NO + NO₂) increased over 20 ppbv throughout the Southern Hemisphere polar lower mesosphere by the Upper Atmosphere Research Satellite (UARS) Halogen Occultation Experiment (HALOE) instrument, and a short-term ozone depletion of 40% in the Southern Hemisphere polar lower mesosphere was measured by the National Oceanic and Atmospheric Administration (NOAA)-16 Solar Backscatter Ultraviolet Instrument (SBUV/2) instrument, probably a result of the HOx increases. Bag et al. [23] used radar data to study the effect of proton events caused by M-level flares on the interlayer sodium layer. Denton [24] used the ozone mineral data from 191 solar proton events to confirm the destructive effect of the Arctic polar vortex on the rapid decline in the ozone level after proton events. Pedatella et al. [25] analyzed the effects of solar flares on the 150 km ionosphere using observational data from Jicamarca and the Whole Atmosphere Community Climate Model-eXtended (WACCM-X). There is, however, little research on the response of the middle atmosphere to the instantaneous enhancement of ultraviolet radiation during a single solar flare event; accordingly, the WACCM-X model is used here for a simulation study.

Solar ultraviolet (UV) radiation at wavelengths of less than 400 nm is an important source of energy for aeronomic processes throughout the solar system [26]. Solar UV photons are absorbed in planetary atmospheres, as well as throughout the heliosphere, via the photodissociation of molecules, photoionization of molecules and atoms, and photoexcitation, including resonance scattering [9]. In a previous study, the Flare Irradiance Spectral Model (FISM) data were used to set the resolution of the 0.01–190 nm band to five minutes; however, the 190.5–399.5 nm observational data were not matched to the minute-scale as a support, so the NASA Goddard Space Flight Center 2nd generation composite Solar Spectral Irradiance day-scale product (GSFCSSI2) [27] were used as the 190.5–399.5 nm model input. The 0.01–399.5 nm data were reconstructed to five-minute resolution in this paper as mode input for the instantaneous UV radiation enhancement that occurred during a flare event to study its impact on the middle and upper atmospheres.

2. Model Description and Methods

2.1. Solar Spectral Irradiance Data

The Flare Irradiance Spectral Model-Version2 (FISM2) [28] was developed at the Laboratory for Atmospheric and Space Physics in Boulder, Colorado. FISM was originally published in 2005 and is composed of “daily” and “flare” products. The “daily” product, consisting of a single daily average spectrum since 1947, and the “flare” product have had 60-s time accuracy since 2003. FISM2 has been upgraded to a 0.1 nm spectral interval and can be used over the full spectral range of 0.1–190 nm. To better fit many ionospheric and thermal models, such as WACCM/WACCMX and TIME-GCM, two other binned daily and flare data products have also been released at the “Stan band” wavelength [19].

FISM2 is based on the NASA Solar Radiation and Climate Experiment (SORCE) X-Ray Sensor (XPS) L4, Solar Dynamics Observatory (SDO) Extreme-ultraviolet Variability Experiment (EVE), and SORCE Solar Stellar Irradiance Experiment (SOLSTICE) data, and these base datasets are related to daily proxies such as F10.7, Mg II c/w, Lyman Alpha, and SDO/EVE/EUV Spectral Photodiodes (ESP) Quad Diode (0–7 nm) for the daily components. The Geostationary Operational Environmental Satellite (GOES)/XRS-B and its derivative are used as proxies for the flare products.

The NASA Goddard Space Flight Center 2nd generation composite Solar Spectral Irradiance product (GSFCSSI2) [27] here are daily average solar spectra over the wavelength range 190.5–399.5 nm in 1 nm bins, covering the time period from 8 November 1978 to 21 November 2019. This data set was created by merging the public irradiance data products from nine different satellite instruments: Solar Mesosphere Explorer (SME), Nimbus-7 Solar Backscatter Ultraviolet (SBUV), NOAA-9 SBUV/2, NOAA-11 SBUV/2, Upper Atmosphere Research Satellite (UARS) Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), UARS SOLSTICE, NOAA-16 SBUV/2, Aura Ozone Monitoring Instrument(OMI), and SORCE SOLSTICE.

$$E{\left(\lambda ,t\right)}_{peak}=E{\left(\lambda ,t\right)}_{daily}\times \left(1+\frac{E{\left(\lambda ,t\right)}_{max}-E{\left(\lambda ,t\right)}_{min}}{E{\left(\lambda ,t\right)}_{min}}\right)$$

(1)

We obtained the peak value of the UV spectral ranges between 190.5 nm and 399.5 nm at the flare peak using Equation (1). $E{\left(\lambda ,t\right)}_{max}$ and $E{\left(\lambda ,t\right)}_{min}$ are the maximum and minimum values of the diurnal variation in the solar irradiance from 1978 to 2019 (see Figure 1), respectively, and $E{\left(\lambda ,t\right)}_{daily}$ is the daily mean value of the solar irradiance in the corresponding band on the day of the simulated event.

The combination of FISM2 and the GSFCSSI2 data were used to build a dataset with a five-minute temporal resolution with irradiances from 0.01 nm to 399.5 nm in the X-ray ultraviolet and ultraviolet spectral ranges (see Figure 2). We ensured that the band from 190.5 nm to 399.5 nm maintained a change trend that was completely consistent with the X-ray ultraviolet during the simulated event.

The solar irradiance changes for the X17 flare (28 October 2003) period are based on the SOLSTICE [29] aboard the SORCE. As expected, the GOES X-ray and XPS 0.1–7 nm measurements have the largest variations, being more than an order of magnitude change. The Solar EUV Experiment (SEE) measurements occurred a few minutes after the peak but still show an increase by a factor of 2 for the EUV range. The H I Lyman α (121.6 nm) emission core and the Mg II h and k emissions (280 nm) show a modest increase of 20% and 12%, respectively. Both are consistent with other FUV emissions measured by the SEE EUV Grating Spectrograph (EGS) [7]. Therefore, we can use the FISM2 and the GSFCSSI2 data as the model input to study the impact of the flare event.

2.2. WACCM-X

WACCM-X is a model of the entire atmosphere that extends into the thermosphere to an altitude of ~500 km and includes the ionosphere. It is a global three-dimensional atmospheric model developed by the Center for Atmospheric Research (NCAR) by researchers in the Geospace section of the High Altitude Observatory, in the Atmospheric Chemistry, Observations, and Modeling Laboratory, and the Climate and Global Dynamics division, as well as external collaborators. It is also a subsystem of the Community Earth System Model (CESM) [30].

WACCM-X involves the cooperation of the three departments of NCAR: the High Altitude Observatory upper atmospheric model TIME-GCM, the Atmospheric Chemistry_middle atmospheric chemical model MOZART, and the Climate and Global Dynamics_middle atmospheric model MACCM. WACCM-X is an extended version of WACCM, extending the height of the model to 2.5 × 10⁻⁹ hPa (~500 km) with a horizontal resolution of 1.9° × 2.5° and a vertical stratification of 88 layers [31]. The WACCM-X version adopted in this paper is a comprehensive numerical model with thermal and ionospheric expansion, covering the height range from the Earth’s surface to the upper thermosphere. WACCM-X is run with a 5-min time step to better resolve the effects of the flare.

2.3. Numerical Experiment

Two simulations were performed for this study (

Table 1. Summary of simulations performed on 18 October 2006.

Case	ssi < 190 nm	190 nm < ssi < 400 nm	ssi > 400 nm	ap	kp
Baseline	Daily	Daily	Daily	10	1
Flare	5-min flare data	5-min simulation data	Daily	10	1

). The background field consists of data from 18 October 2006 under solar minimum conditions. The first simulation was a baseline simulation in which no forcing from solar proton events or flares was applied. The second simulation was forced by a flare, and the flare data were recorded on 28 October 2003 and were used as flare input for 18 October 2006. To best capture the effects of the solar flare, both WACCM and the input data needed to have a high temporal resolution. The reduced time step in the model was coupled with high time resolution solar data from FISM2 [32,33]. For the flare simulations included in this study, the FISM2 flare data were processed into 5-min averages at 23 wavelengths below 121 nm and at 1 nm resolution above 120.5 nm. The peak values between 190.5 nm and 399.5 nm were calculated using the diurnal variation in the FISM daily data over several years. For the baseline simulation, the data consisted of the daily averages on 18 October 2006. The importance of the high time cadence of the flare input is illustrated in Figure 2.

3. Results and Discussion

3.1. Atmospheric Ozone Differences between the Flare Simulation and Baseline Simulation

Figure 3 illustrates how the photochemistry and heating of the atmosphere vary with altitude and the wavelength of the solar radiation. Nominal subdivisions of the UV spectral range are as follows: near ultraviolet (NUV) from 300 to 400 nm, middle ultraviolet (MUV) from 200 to 300 nm, far ultraviolet (FUV) from 120 to 200 nm, extreme ultraviolet (EUV) from 30 to 120 nm, X-ray ultraviolet(XUV) from 1 and 30 nm, and X-rays at wavelengths less than 1 nm [9]. The EUV and XUV mainly influence the upper mesosphere and lower thermosphere. Solar MUV radiation heats the stratosphere, while solar UV radiation shortward of 170 nm heats the thermosphere. Atmospheric absorption of solar UV radiation also initiates many chemical cycles, such as involving water vapor, ozone, nitric oxide, and chlorofluorocarbons in Earth’s atmosphere. The longer wavelengths of the solar radiation, mainly from the NUV, visible (VIS: 400–700 nm), and near-infrared (NIR: 700–10,000 nm) spectral regions, are absorbed (and transmitted and reflected) by aerosols, clouds, and gases in the troposphere and by land surfaces and ocean [9].

Figure 4a–e illustrates the relative differences in the atmospheric ozone between the flare simulation and baseline simulations at different latitudes. Figure 4a–c is for the different times of flare at 0° E; Figure 4a shows the start of flare at 11:00:00 UT; Figure 4b is after 30 min of flare start at 11:30:00 UT; Figure 4c is after 1 h of flare start at 12:00:00 UT, and the flare is at its end. Figure 4d shows the relative differences in the atmospheric ozone between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere. Figure 4e shows the relative differences in the atmospheric ozone between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 10:00:00 UT (1 h before flare start) to 14:00:00 UT (2 h after flare end) on 18 October 2006 in the middle atmosphere.

Figure 4a shows that the deviation of ozone in the middle atmosphere is approximately 0 at all latitudes before the start of the flare, which means the flare case is the same as the baseline case before the start of the flare. Figure 4b demonstrates the situation after 30 min of flare at 11:30:00 UT; the figure shows that, in the stratosphere, there is an 0.6% increase in ozone at 40–50 km in the low latitudes in the Northern Hemisphere, and the ozone response of the overall Southern Hemisphere increases by 0.6% at the same altitudes. O₃ in the upper stratosphere extends from the tropical high-value center to the Arctic (Antarctic) as a result of large-scale air movements such as Brewer–Dobson circulation [34], which forces the stratospheric ozone from the tropics to the polar regions. The ozone layer can absorb most of the UV rays (mainly wavelength at 200–320 nm). The ozone generation reaction and the reaction rate increase by the largest magnitude compared with other ozone-depleting reactions when the solar UV radiation is enhanced, resulting in an overall ozone increase in the stratosphere. In the mesosphere, there is a positive response of up to 6% at 70–85 km, except in high latitudes in the Northern Hemisphere. In the low thermosphere, there is a −3.6% response above 85 km in all latitudes. At 70–85 km, there is insufficient HOx production to overcome increased Lyman-α photodissociation of molecular oxygen (odd hydrogen still plays a role in determining the ozone density to about 82 km). Thus, at 85 km, where increased net production of odd oxygen maximizes, there is a peak in the ozone response.

Figure 4c demonstrates the situation after 1 h of the flare, starting at 12:00:00 UT, when the flare ends. The figure also shows there is a 0.6% increase in ozone at 40–50 km in the stratosphere except in the high latitudes in the Northern Hemisphere. In the mesosphere, there is a smaller positive response at 85 km in the low and middle latitudes of the Southern Hemisphere. In the low thermosphere, there is a negative response of 1.2%, which is smaller than that at 11:30:00 UT.

Figure 4d shows the relative differences in the atmospheric ozone between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere. The figure fully shows the ozone response process of the stratosphere before and after the flare event; the average enhancement of the MUV by 9% in one hour causes the stratospheric ozone in low latitudes to increase by 0.6% for more than one day, which is weaker than the previous study. Keating et al. [11] found that the stratospheric ozone response with no correction for temperature effects is found to be approximately a 0.4% increase at 3 mbar for a 1.0% increase in 205-nm solar radiation based on the Nimbus 7 data. The ozone response in high Southern latitudes is similar to the previous ones [15]. Pikulina et al. [15] found the stratospheric ozone response is about within 1% at 50°~70° S and about within 5% at 75°~88° S at 50 km; the result in the stratosphere is weaker as a result of MUV (200–300 nm), while the previous result is under the solar radiation from 0 to 190 nm.

In the mesosphere, the ozone response mainly occurred during the flare event, so Figure 4e shows the relative differences in the atmospheric ozone between the flare simulation and baseline simulations at low latitudes (30° S–30° N) during the flare event from 10:00:00 UT (1 h before flare start) to 14:00:00 UT (2 h after flare end) on 18 October 2006. There is a positive response of ozone in the stratosphere and at 70–75 km in the mesosphere, which is 0.6% and 4%. However, there is a −1.2% response at 50–65 km during the flare. In the low thermosphere, there is a −6% response above 85 km. As the mesospheric ozone is currently believed to respond to solar UV irradiance variations in the wavelength range 120 nm < λ < 300 nm and the photochemical time constant for changes in ozone concentration in the mesosphere is short, ozone there is highly variable. In the region between 55 and 70 km, the increased Lyman-α solar flux enhances the photodissociation of water vapor and hence increases the density of odd hydrogen (HOx), resulting in a decrease in ozone [10].

3.2. Atmospheric Temperature Differences between Flare Simulation and Baseline Simulation

Figure 5a–d illustrates the differences in the atmospheric temperature between the flare simulation and baseline simulations at different latitudes. Figure 5a–c illustrates the different times of flare at 0° E. Figure 5a shows the start of the flare at 11:00:00 UT; Figure 5b demonstrates the situation 30 min after the start of the flare at 11:30:00 UT; Figure 6c demonstrates the situation 1 h of the flare started, at 12:00:00 UT, when the flare is ending. Figure 5d shows the differences in the atmospheric temperature between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere.

Figure 5a shows the deviation of temperature in the middle atmosphere is approximately 0 at all latitudes at the start of the flare, and it is convincing, which means the flare case is the same as the baseline case before the start of the flare. Figure 5b demonstrates the situation 30 min after the start of the flare at 11:30:00 UT; the figure shows there is an 0.05 K increase in temperature at 40–50 km in the stratosphere in the low latitudes of the Southern Hemisphere and Northern Hemisphere, while in the middle latitudes, the increase in temperature is in the Southern Hemisphere. In the mesosphere, there is also an 0.05 K increase in temperature at 80–90 km in low latitudes of the Southern Hemisphere. In the low thermosphere, there is also an increase in temperature in the low and middle latitudes larger than in the middle atmosphere. Solar flares produce large and rapid increases of solar X-ray and extreme ultraviolet (EUV) radiation. These rapid increases directly enhance ionization in the upper atmosphere, causing “sudden ionospheric disturbances” (SID) [35]. These temperature differences, which are caused by the heating of the thermosphere due to enhanced ionization, are altitude-dependent.

Figure 5c shows the status 1 h after the flare started, at 12:00:00 UT, when the flare is ending. The figure also shows there is an 0.05 K increase in temperature at 40–50 km in the stratosphere in the Southern Hemisphere and in low and middle latitudes in the Northern Hemisphere. In the mesosphere, there is also an 0.05 K increase in temperature at 70–80 km in low and middle latitudes of the Southern Hemisphere. In the low thermosphere, there is also an increase in temperature in the low and middle latitudes larger than in the middle atmosphere. Figure 5d shows the differences in the atmospheric temperature between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere. The figure fully shows the temperature response process of the stratosphere before and after the flare event. The average enhancement of the MUV by 9% in one hour causes the stratospheric temperature in low latitudes to increase by 0.05 K for more than two days, which is weaker than the previous study, in which the calculated short-term increase in temperature at 2 mb for a 1% increase in 205 nm radiation (and corresponding 0.09% increase from 270.3 nm to 317.5 nm) was only 0.05 K [11].

3.3. Atmospheric Density Differences between the Flare Simulation and Baseline Simulation

Figure 6 is analogous to Figure 5 but shows the differences in the relative atmospheric density. Figure 6a–e illustrates the differences in the relative atmospheric density between the flare simulation and baseline simulations at different latitudes. Figure 6a–c shows the different times of the flare at 0° E. Figure 6a shows the start of the flare at 11:00:00 UT; Figure 6b demonstrates the situation 30 min after the start of the flare at 11:30:00 UT; Figure 6c demonstrates the situation 1 h after the start of the flare, at 12:00:00 UT, when the flare is ending. Figure 6d shows the differences in the relative atmospheric density between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere.

Figure 6a shows the deviation of relative atmospheric density in the middle atmosphere is approximately 0 at all latitudes at the start of the flare, and it is convincing, which means the flare case is the same as the baseline case before the start of the flare. Figure 6b demonstrates the situation 30 min after the start of the flare, at 11:30:00 UT; the figure shows there is an 0.05% increase in relative atmospheric density in the low latitudes of the Southern Hemisphere and Northern Hemisphere, while there is an 0.1% increase in relative atmospheric density in the Southern Hemisphere in the middle latitudes. In the mesosphere, there is also an 0.05% increase at 80–90 km in low latitudes of the Southern Hemisphere.

Figure 6c demonstrates the situation after 1 h of the start of the flare, at 12:00:00 UT, which is when the flare is ending. The figure also shows there is an 0.05% increase at 60–70 km in the Southern Hemisphere and in low and middle latitudes in the Northern Hemisphere. In the mesosphere, there is also an 0.15% increase at 80–90 km in the middle latitudes of the Southern Hemisphere. In the low thermosphere, there is also a 0.1% increase in the low and middle latitudes.

Figure 6d shows the differences in the relative atmospheric density between the flare simulation and baseline simulations at low latitudes (30° S–30° N) from 18 October 2006 to 20 October 2006 in the middle atmosphere. There is a 0.05% increase above 60 km, and there is a 0.15% increase at 80–90 km. The reason may be that the increase in temperature in the stratosphere heats the atmosphere, causing the density to increase due to atmospheric lift. The duration of the increase in density is consistent with the duration of the increase in stratospheric temperature for more than two days.

4. Conclusions

We reconstructed the solar radiation data in the 0.5–399.5 nm band with 5-min resolution using FISM2 and GSFCSSI2 data as input to WACCM-X to simulate the response process of the middle atmosphere. We conducted a UV radiation enhancement study by setting the flare event level during a solar quiet period. The results indicate that, within 1 h of the UV radiation enhancement, an ozone-led photochemical process caused corresponding changes in the atmospheric temperature and density; however, the response heights and latitudes were different for these changes. The solar radiation data in the 0.5–190.5 nm band were reconstructed from various satellite data, and the results of previous simulation studies compared with the observations during the flare period were thoroughly verified. The solar radiation data in the 190.5–399.5 nm band are based on decades of solar radiation day changes, which may include deviations from the actual flare period changes. The simulated data in this study are consistent with the amplitude of the flare changes in the observed data. The reconstruction method used in this paper is designed to study the impact on the middle atmosphere of UV radiation when it is instantaneously enhanced, and the results confirm that the middle atmosphere has an immediate response to the UV radiation, though the response of temperature and ozone in different altitudes is weaker than the previous study.